Conventionally, when a covering layer of elastomeric material is applied to a core of indefinite length, the layer is extruded onto the core. In the manufacture of electrical cable, an insulation layer is formed in this way by extrusion apparatus operating with a single extrusion head. Particularly in the case of telecommunications cables, electrical conductors may each be provided individually with two layers of elastomeric insulation, i.e. an inner and an outer layer, which provide different electrical properties. For the purpose of forming two layers of insulation, it is now conventional practice to employ the use of an extruder head known as a "dual" extruder head which is supplied with extrudate for the two layers from two different extruders each connected to an individual inlet of the dual extruder head.
There are practical problems in connecting two extruders directly to a dual extrusion head. It has been found that once such an assembly is in use, the effects of the different rates of expansion and contraction of the parts of the assembly and caused by the molten extrudate or from the barrel heaters result in distortion and cracking of the head and perhaps of the extruders. In order to overcome this problem in a practical way, it is conventional practice to connect one extruder directly to the head, the other extruder being spaced from the head and connected to it by a flexible transfer tube for the extrudate. A problem with the use of a transfer tube is that the temperature of the extrudate (particularly polyvinylchloride) as it passes along the transfer tube is difficult to control even with the use of heaters around the tube, and burning of the extrudate easily results. In a case where the extrudate passing through the transfer tube is for providing a foam layer upon a conductor, difficulty has been found in controlling the volume of air space within the finished layer. Such an air space is sometimes referred to as "percentage blow".